ADDRESS. 19 



margins of fclie ocean are necessarily those of greatest folding and conse- 

 quent elevation. We have thus a hard, thick, resisting ocean-hottom 

 which, as it settles down toward the interior, under the influence of 

 gravity, squeezes upward and folds and plicates all the soft sediments 

 deposited on its edges. The Atlantic area is almost an unbroken cake 

 of this kind. The Pacific area has cracked in many places, allowing the 

 interior fluid matter to ooze out in volcanic ejections. 



It may be said that all this supposes a permanent continuance of the 

 ocean-basins, whereas many geologists postulate a mid- Atlantic continent • 

 to give the thick masses of detritus found in the older formations both in 

 Eastern America and Western Europe, and which thin ofi" in proceeding 

 into the interior of both continents. I prefer, with Hall, to consider these 

 belts of sediment as in the main the deposits of northern currents, and de- 

 rived from Arctic land, and that like the great banks of the American coast 

 at the present day, which are being built up by the present Arctic current, 

 they had little to do with any direct drainage from the adjacent shore. 

 We need not deny, however, that such ridges of land as existed along the 

 Atlantic margins were contributing their quota of river-borne material, 

 just as on a still greater scale the Amazon and Mississippi are doing now, 

 and this especially on the sides toward the present continental plateaus, 

 though the greater part must have been derived from the wide tracts of 

 Laurentian land within the Arctic Circle or near to it. It is further 

 obvious that the ordinary reasoning respecting the necessity of continental 

 areas in the present ocean basins would actually oblige ns to suppose 

 that the whole of the oceans and continents had repeatedly changed 

 places. This consideration opposes enormous physical difficulties to any 

 theory of alternations of the oceanic and continental areas, except locally 

 at their margins. I would, however, refer you for a more full discussion of 

 these points to the address to be delivered to-morrow by the President of 

 the Geological Section. 



But the permanence of the Atlantic depression does not exclude the 

 idea of successive submergences of the continental plateaus and marginal 

 slopes, alternating with periods of elevation, when the ocean retreated 

 from the continents and contracted its limits. In this respect the Atlantic 

 of to-day is much smaller than it was in those times when it spread 

 widely over the continental plains and slopes, and much larger than it 



' Among American geologists, Dana and Le Conte, though from somewhat differ- 

 ent premises, maintain continental permanence. Crosby has argued on the other 

 side. In Britain, Hull has elaborated the idea of interchange of oceanic and conti- 

 nental areas in his memoir in Trans. Dublin Society, and in his work entitled The 

 Physical History of the British Islands. Godwin- Austen argues powerfully for the 

 permanence of the Atlantic basin, Q. J. Geol. Society, vol. xii. p. 42. Mellard 

 Eeade ably advocates the theory of mutation. The two views require, in my judg- 

 ment, to be combined. More especially it is necessary to take into account the 

 existence of an Atlantic ridge of Laurentian rock on the west side of Europe, of 

 which the Hebrides and the oldest rocks of Wales, Ireland, Western France, and 

 Portugal are remnants. 



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